In the Specification of my British Pat. No. 1,376,941 there is described a method of reproducing a three dimensional form from a photographic image of that form, comprising the steps of, projecting a series of uniformly spaced parallel lines onto the object to be reproduced, photographing the object together with the superimposed lines, projecting an image of the photograph so taken onto a screen, tracing out each of the superimposed contour lines on the image on the screen in turn by way of a stylus linked by way of a pantograph to a cutting tool which is arranged to cut out a corresponding contour on a block of material to be formed into said three dimensional form wherein the said block of material is advanced both horizontally forwards and vertically downwards with respect to the plane of movement of said cutting tool before each new contour line is traced out by said stylus.
As mentioned in the Specification of my British Pat. No. 1,376,941, errors arise using the apparatus described and which must be corrected or compensated for to ensure that the final reproduction of the three dimensional form is an accurate representation of that form.
The first source of error arises from the fact that due to the necessarily finite distance between the object and the projector which projects the series of parallel lines onto the object, a spreading out of the light rays takes place and the spacing of the projected lines superimposed on the object will be slightly greater at points on the object which are further from the projector, than at points which are nearer the projector. Following from this, a further error arises due to the fact that the plane of each projected line is not parallel to adjacent lines and each contour cut will not be accurate unless the block of material being cut is rotated about a horizontal axis to compensate for this. A first object of the present invention is to remove this source of error. Apart from the errors just discussed for the projector, which also apply to the camera, a second source of error arises from the fact that for the lines to appear as accurate contour lines on the photograph taken of the object together with the superimposed lines, it is necessary for the camera to view the object at an angle of 90.degree. to the object-projector axis. This is not satisfactory in practice as the resolution between the lines on parts of the object almost perpendicular to the projector--object axis would be too small for each to be correctly identified in the subsequent stage of the method. As described in my earlier patent an angle of 36.degree. is used in place of the theoretical 90.degree. and a compensation for this angle is made in a subsequent stage in the process.
In my earlier patent, this compensation is achieved mechanically by linking the sideways movement of the cutting tool to a compensatory sideways movement of the block of material as it is cut by the tool. It is a second object of the present invention to produce this compensation optically so eliminating the complication necessary to produce the mechanical compensation described in my earlier patent.